Putting the spare under the caravan adds the weight of the carrier and a 50/50 chance that when you need it, the carrier will slide out on the side which is punctured and be very difficult to get at. Get a cover for the wheel, and put the caravan spare in the tow car while you are travalling - easy to get to and you have a cover for the dirty wheel you take off. Once on site put the spare under the van locking it to one of the legs with a cycle type anti theft device if you wish,